While the invention may find application in many electrical power control situations (e.g., lamps, heaters, motors, etc.), the following discussion refers particularly to the control of the dimming of lamps such as in a theatrical lighting system. Phase controlled dimming systems using semiconductors to switch the load current on and off are well known. This type of circuit is known for its efficiency and effectiveness for the purpose. Also known are several disadvantages.
Electromagnetic interference (EMI) from the dimmer circuitry and power lines, and audible noise from lamp filaments are associated with the rapid changes in load voltage and current. Dimmers which employ rapidly switched power control elements require inductance and/or capacitance to slow down the rate of current change in the load to minimize this interference and noise. The longer the transition time between no current flow and full current flow, the less objectionable these effects are. Increasing this switching time with chokes require larger and lossier inductors to decrease these effects. Previous chokeless dimmers which increase the switching time by turning the power switch element itself on or off at a relatively slow rate require rise times similar to those obtained by use of chokes. These long rise times lead to higher switching component temperatures and less efficiency in the dimmer. We propose improvements which allow control of the shape of the rise time waveform to provide excellent noise reduction while reducing the risetime for increased efficiency and less heat generation.
A semiconductor power switch is susceptible to damage or destruction from excessive current flow, especially when a transistor is used instead of a thyristor. Previous dimmers have used various methods to monitor current flow, but have required that the current exceed the maximum allowable for the dimmer before triggering an overload condition. This does protect the power switch adequately, but compromises must be made. If the shutdown circuit must be reset manually after tripping, the overload current threshold must be set very high compared to the long-term current in order to avoid nuisance tripping due to cold lamp filaments. If the shutdown circuit resets automatically (perhaps every half-cycle), then large repetitive currents will flow each time power is applied, until the current trip point is reached after each power application. By using the signals from the load current sensing in new ways, improvement are possible to provide features beyond protection.
Unwanted variations in load voltages are very undesirable in many situations. Lighting designers in theaters take great pains to set light levels exactly for various effects, and even relatively small brightness changes with incandescent lamps cause variations in color temperature which are extremely troublesome for television, movie and photography studios.
These variations have several sources. Changes in AC power line voltage cause the power delivered to the load to change, leading to noticeable brightness variations in lamp loads. In addition, the long wire runs used in many lighting applications between the dimmer and the load are responsible for voltage drops which cause a high-wattage lamp to receive less voltage than a low-wattage lamp at a given dimmer output. Previous dimmers which allow individual adjustment for each output connection have required calibration for each different load at each outlet, and re-adjustment of the dimmer is required whenever the load is to be changed.
Acoustic noise from lamp filaments is undesirable in many dimming applications, especially television and movie studios. The changes in current which are typical of phase-control dimmers cause lamp filaments to vibrate. The more rapidly the current changes, the more noise is produced. Phase-control dimmers, both choke-type and previous chokeless designs, slow down the transition time between full current flow and no current flow to minimize this filament noise.
The inductance of choke-type dimmers slows down the current change, and thereby reduces the filament noise. However, this method of noise reduction has several drawbacks. First, the shape of the current waveform depends on the amount of current flowing through the load, so the amount of quieting varies with changes in the load. Second, the shape of the current waveform is dependent on the load and the choke, and is not adjustable. Third, the chokes required for this purpose are large, heavy and expensive.
Chokeless phase-control dimmers using transistor control the switching of the power semiconductors so they are, for a time, operating in their linear mode rather than full-on or full-off. However, the greatest amount of heat is dissipated during this "linear mode" switching time. For the multi-kilowatt level, the power switching devices increase greatly in size and cost. In addition, the heat dissipation required of the power switch increases much faster than the current it controls because the voltage drop across the switch tends to increase with increased current flow. Also, the heat dissipation is concentrated, requiring the use of expensive custom heat sinks and hardware, and limiting opportunities for using the dimmer in a variety of configurations.
Because transistors only control current in one direction, a full-wave dimmer must make provisions for current routing in the proper direction for each switch. This can be done by connecting certain semiconductors (such as FETs) in inverse series, so the internal diode formed by the transistor construction passes current in the reverse direction to the transistor's control direction. Transistors can also be connected in inverse parallel (with blocking diodes, if necessary). However, either of these approaches require the use of twice as many expensive transistors as would be indicated by the amount of current flow.
Another approach, as shown in our prior U.S. Pat. No. 4,949,020, is to use a diode bridge to present only rectified AC to the semiconductor switch, and to parallel switch transistors as needed for the current required. This offers lower cost as diodes are cheaper than transistors, and simpler control, because only one output drive control circuit is needed. This approach works well up to about 3,000 watts load. Above this level, the advantages of the diode bridge method diminish because both localized heat dissipation and diode cost start to rise rapidly.
The ability to use inexpensive diodes in parallel to obtain higher amperage handling, or to connect dimmer sections in parallel for higher amperage, more flexibility, or N+1 redundancy (for increased reliability in critical situations, a load which would require N dimmer sections can be powered by N+1 sections; if one section fails, the load can continue to operate) would be a great improvement in flexibility, cost reduction and reliability.